Conductive plastic compositions have been well received as desirable raw materials for the fabrication of a variety of specialized accessories and components, including static electricity dissipation devices, electrical heating elements, equipment parts for high frequency protection and/or electromagnetic interference (EMI) shielding and a variety of other electrical components such as electrodes, terminals, connectors, and the like.
Thermosetting or heat-curable polymer systems have been most prominent in the majority of such conductive plastics materials which have been developed so far. For certain electrical applications, the resistance of many thermosetting materials to high temperature service conditions is a major consideration. However, a generally more important factor probably resides in the inherent reactivity responsible for their thermosetting character and which tends to increase the polymeric interaction with the finely subdivided conductive solids (e.g., metallic powders, carbon blacks, and the like) that must be incorporated into the polymeric base material in order to provide appropriate levels of electrical conductivity.
Most thermoplastic resins, on the other hand, are considerably less responsive to additions of finely divided solid fillers, often resulting in an actual deterioration of many structurally significant physical properties when filled with carbon blacks, powdered metals, and the like, to the extent required for practical levels of electroconductivity. Such deficiencies have severely limited applications accessed by conductive thermoplastic compositions, confining them for the most part to fabrication of at least partially supported auxiliary elements and secondary components like seals, gaskets, inserts and electrodes.
In spite of such difficulties, filled thermoplastic systems have, of course, continued to receive attention since rigid thermoplastic resins offer definite advantages over most thermosetting materials in regard to ease of handling, melt processing convenience and the simplicity of fabricating finished articles therefrom by the usual high speed plastic forming techniques such as extrusion, injection molding, and the like. Indicative of approaches which have been taken in an effort to develop metal-filled thermoplastic compositions with improved overall performance and utility are those disclosed in the publications summarized below.
U.S. Pat. No. 3,491,056 to Saunders et al discloses the rare ability of finely divided aluminum powder to strengthen a specialty thermoplastic resin derived from the prescribed copolymerization of ethylene with an unsaturated carboxylic acid such as acrylic acid. It appears, however, that outstanding levels of electrical conductivity were not achieved in this system even with a 50% by volume loading of conductive filler unless some of the fine aluminum powder was replaced with carbon black (e.g., 16% by volume as in Example 7).
U.S. Pat. No. 3,867,315 to Tigner et al is much more concerned with achieving good electrical conductivity levels without excessive volume loadings of the particular metallic filler material. This is accomplished by including various ionic metal salts along with the metallic filler, which is either copper or contains accessible copper. A broad list of thermoplastic resins is recited, but experimental data is presented only for a blend of 2 parts polyethylene with 1 part of a 72/28 copolymer of ethylene and vinyl acetate, and no physical strength properties whatsoever are indicated. A closely related patent is U.S. Pat. No. 3,919,122 to Tigner which deals with substantially the same system except that the ionic salt is a metal halide salt which is formed "in situ" from free metal and a suitable halide source. The preferred halide source is a halogen-containing polymer (notably one derived from vinylidene chloride), with a copolymer of vinyl chloride and vinylidene chloride in respective weight proportions of 27:73 being used in most of the illustrative examples. However, the only metallic filler used in said examples is a brass powder with an average particle size of 5 to 12 microns and, again, no physical strength measurements are presented.
U.S. patent application Ser. No. 238,757 filed Feb. 27, 1981, now abandoned, to Kleiner describes a flame retardant thermoplastic filled with anisometrically shaped aluminum particles in amounts of from 12 to about 40% by volume which exhibit good electroconductivity. However, the ability to maintain the electroconductivity while lowering the amount of aluminum particle filler needed to achieve the desired level of electroconductivity through the addition of a hard low aspect ratio filler is not realized.
Another approach to achieving highly conductive metal-filled plastic composites at very low volume loadings of the metallic filler has been resorted to from time to time in this art. The basics of this approach, which is often referred to as the "segregated metal particle network" technique, is the careful observance of several critical processing conditions in fashioning the finished composite. These conditions generally include dry mixing of rather large granules of organic polymer with much smaller particles of metal and compacting the resulting mixture under pressures and temperatures controlled to cause some coalescence or sintering between neighboring polymeric granules without effecting sufficient melt flow to result in extensive intermingling with the fine metallic particles distributed therebetween. By means of such techniques, highly conductive, compacted metal-polymer composites can be obtained at metal filler loadings below about 10% by volume, due to the resulting preferential segregation of metal particles into extended chain-like networks which apparently serve as a system of three-dimensionally interconnected pathways through which current can flow. Patents describing products made by such techniques include U.S. Pat. Nos. 2,761,854 to Coler and 3,708,387 to Turner et al. Additional descriptions are also found in the basic research literature, including such recent journal articles as:
Journal of Applied Polymer Science 20, pp. 25752580 (1976) by Mukhopadhyay et al and Polymer Engineering and Science 19, pp. 533-544 (1979) by Bhattacharyya et al.
Unfortunately, industrial applications for said products appear to be extremely limited since the associated techniques are totally abhorrent to the high speed, "fused state" mixing and molding operations for which thermoplastic materials are so well suited and for which reason they are usually selected in commercial practice. Furthermore, in view of the inherent heterogeneous nature of such "segregated network" metal-polymer compacts, it is very doubtful that adequate manufacturing uniformity and reproducibility could be achieved for commercial articles except possibly those of the simplest shape and design and least demanding fields of application.
In view of the apparent state of this art, a considerable need continues to exist for improved and more versatile metal-filled polymeric compositions. One of the most challenging raw material requirements in this field resides in the need for conductive thermoplastic molding compounds suitable for forming flame retardant structural members of sufficient size, mass and complexity to serve as electronic cabinet housings, dampers and/or shields for absorbing or blocking out electromagnetic field effects or other high frequency electrical emissions. The region of high frequency generally addressed is that region referred to as the radio frequencies, although protection from interference in this region as well as in the microwave frequency region can also be achieved.
Thus, for example, the computer and auto industries have set guidelines which indicate that materials suitable for cabinet housings and having a shielding effectiveness (SE) of 20 to 30 dB will apparently meet 50% of their needs, while an SE of 30 to 40 dB will apparently meet 95% of their needs. Shielding effectiveness is an absolute ratio normally expressed in decibles (dB) and defined on a logarithmic scale through the following equations SE=20 log (Ei/Et) or SE=10 log (Pi/Pt) where E is the field strength in volts per unit length, P is the field strength in watts per unit area, i is the incident field and t is the transmitted field. Alternatively, SE can also be expressed on a linear scale as a percent attenuation (PA). PA is simply (Ei/Et).times.(100) or (Pi/Pt).times.(100). Thus, 99% attenuation corresponds to 20 dB, 99.9% to 30 dB and 99.99% to 40 dB. Finally, it should be pointed out that there is often a crude correlation between the shielding effectiveness and the volume resistivity, such that a volume resistivity of lower than 6 ohm-cm usually insures that the shielding effectiveness will be at least 20 dB.
It is also understood, however, that this level of shielding effectiveness is not needed for "anti-static" applications and, therefore, lower levels of protection will suffice, for example, volume resistivity levels of less than 1.times.10.sup.8 ohm-cm or surface resistivities below approximately 10.sup.9 ohm/square.
Accordingly, a goal of the present invention is to provide compounds of high electroconductivity. A more specific objective of this invention is to formulate thermoplastic molding compounds of exceptional levels of electroconductivity, while reducing the total amount of metallic filler needed to achieve the desired electroconductivity characteristics. Such molding compounds are particularly needed for certain specialized structural uses, such as EMI shielding members, electronic equipment housings, and the like, and thus represent a preferred embodiment of the present invention.